HYDROGEOLOGY OF THE UNCONFINED AQUIFER AT THE U.S. DEPARTMENT OF ENERGY HANFORD SITE, WASHINGTON STATE

The Hanford Site, a facility in the U.S. Department of Energy (DOE) nuclear weapons complex located in southeastern Washington State, was acquired by the federal government in 1943. Until the 1980s, the Site was dedicated primarily to the production of plutonium for national defense and waste management. Groundwater in the unconfined aquifer generally flows from recharge areas west of the Hanford Site and discharges to the Columbia River. The natural pattern of groundwater flow was altered by discharge of large volumes of wastewater to the ground. During the first 50 years of operations, 400 to 450 billion gallons of water, often contaminated, were released to the soil, primarily via ponds and cribs. This resulted in groundwater mounding up to 24 meters in some portions of the site and created highly transient groundwater conditions. Contaminants were also discharged to the unconfined aquifer by other means including retention trenches, injection wells, and waste disposal in pits, trenches, and landfills. High-level radioactive wastes are stored in 177 tanks at Hanford, some of which have leaked through the vadose zone and impacted groundwater. Approximately 200 square kilometers of groundwater in the unconfined aquifer is contaminated above drinking water standards, including plumes of tritium, iodine-129, nitrate, carbon tetrachloride, trichloroethene, chromium, strontium-90, technetium-99, and uranium. The distribution of these contaminants is controlled by past waste management operations, groundwater flow, and complex interactions with the Site geology. Most wastewater discharge was terminated in the mid 1990s, causing a decline in the mounded water table and furthering the complexity of the groundwater flow system and contaminant migration. DOE has monitored groundwater quality across the Hanford Site since operations began, and at six of the groundwater plumes, interim remedial actions are underway. These actions include: pumping and treating plumes; reactive barriers; and monitoring. Research and development efforts and modeling are underway to develop final cleanup decisions for these groundwater contaminant plumes.